Get ready for your brain to hurt. Again.

A math teacher in Vietnam challenged his third-grade class (read: 8-year-old students) to solve this seemingly simple arithmetic problem.

Apparently you're supposed to plug in the numbers 1 through 9, using each digit once, in each of the blank spaces. Using the identified operations and the inserted values, a correct answer would lead to a solution of 66 (those double dots stand for division, by the way).

Sound easy? Try it.

Teacher Tan Phuong gave the puzzle to his students in the town of Bao Loc, according to local outlet VN Express. He said he also sent the exercise to a person with "a doctorate in economics with mathematics" who has yet to return with a solution, The Guardian reported.

So, what's the answer?

There are other possible solutions, but the first one I came up with was: 6, 9, 3, 5, 2, 1, 7, 8, 4 in that order ... How I got to it was assigning each blank to a letter and writing the whole thing as an equation then grouping like terms.So...

a+13(b/c)+d+12e-f+(gh/i)-21=66.Then you add 21 to both sides and a+d-f+13(b/c)+12e+(gh/i)=87. And since you’re limited to plugging in numbers between 1-9 for a-i, you see that there’s only so many combinations that will yield numbers close to 87. So then you start guessing and checking how large you have to make the various numbers to make it work. You need smaller numbers in the denominators and the subtraction and larger numbers being multiplied.

The Guardian's Alex Bellos has promised to upload a solution "later," but if anyone has an easier approach, leave it in the comments.

Phew.